Oxidative processes are a major contributing factor in senile cataracts. W have demonstrated that metal catalyzed oxidation of the crystallins induces protein modifications that mimic those seen in aging, senile cataracts, and brunescent lenses. The importance of the role of metal in oxidative processes related to cataract is further supported by studies in other labs The lens contains high levels of thiols such as glutathione that can participate in these metal-catalyzed oxidation reactions. Our lab has continued our studies on both the interaction of metals with crystallins an the mechanisms that protect the lens against deleterious oxidation reaction A protein that protects enzymes specifically against inactivation by thiol- dependent metal-catalyzed reactions has been reported in yeast and rat tissues. (This activity is not related to catalase, glutathione peroxidase or superoxide dismutase.) Our lab demonstrated the presence of a similar activity in lenses of bovine, guinea pig, human, pig, monkey, and rat. The antioxidant activity consistently copurifies with a subpopulation of glutathione S-transferase mu. Copper, zinc, and iron, but not calcium, induced aggregate formation in bovine lens extracts and solutions of the crystallins. Aggregation, measur by light scattering, was time dependent, occurring at metal-to-protein rati greater than one and varying, depending on the metal and protein. Zinc induced the aggregation of beta- and - but not gamma-crystallin. The affinity of copper and zinc for these proteins is relatively low. The addition of EDTA, DETAPAC, L-histidine, or L-cysteine prevented zinc- and copper-induced protein aggregation and caused complete disaggregation. The treatment of alpha- and beta-crystallin and trypsin inhibitor with diethylpyrocarbonate prevented aggregation induced by zinc but not by copper. The presence of salt decreased the metal-induced aggregation of alpha- and beta-crystallin. No metal-induced changes in secondary and tertiary structures of these proteins were observed by fluorescence and circular dichroism spectroscopy.
